.\"     $OpenBSD: file.9,v 1.15 2015/05/07 08:53:33 mpi Exp $
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.\" Copyright (c) 2002 Artur Grabowski <art@openbsd.org>
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.Dd $Mdocdate: May 7 2015 $
.Dt FILE 9
.Os
.Sh NAME
.Nm file
.Nd an overview of file descriptor handling
.Sh SYNOPSIS
.In sys/file.h
.In sys/filedesc.h
.Ft int
.Fn falloc "struct proc *p" "struct file **resultfp" "int *resultfd"
.Ft int
.Fn fdrelease "struct proc *p" "int fd"
.Ft void
.Fn FREF "struct file *fp"
.Ft int
.Fn FRELE "struct file *fp" "struct proc *p"
.Ft struct file *
.Fn fd_getfile "struct filedesc *fdp" "int fd"
.Ft int
.Fn getsock "struct proc *p" "int fd" "struct file **fpp"
.In sys/file.h
.In sys/filedesc.h
.In sys/vnode.h
.Ft int
.Fn getvnode "struct proc *p" "int fd" "struct file **fpp"
.Sh DESCRIPTION
These functions provide the interface for the UNIX file descriptors.
File descriptors can be used to access vnodes (see
.Xr vnode 9 ) ,
sockets (see
.Xr socket 2 ) ,
pipes (see
.Xr pipe 2 ) ,
kqueues (see
.Xr kqueue 2 ) ,
and various special purpose communication endpoints.
.Pp
A new file descriptor is allocated with the function
.Fn falloc
and freed with
.Fn fdrelease .
.Fn falloc
and
.Fn fdrelease
deal with allocating and freeing slots in the file descriptor table,
expanding the table when necessary and initializing the descriptor.
It's possible to do those things in smaller steps, but it's not
recommended to make complicated kernel APIs that require it.
.Pp
The files are extracted from the file descriptor table using the
functions
.Fn fd_getfile
.Fn fd_getfile
performs all necessary checks to see if the file descriptor number is
within the range of file descriptor table, and if the descriptor is
valid.
.Pp
The files are extracted from the process context using the
function
.Fn getsock
and
.Fn getvnode .
These functions are special cases that besides doing
.Fn fd_getfile
also check if the descriptor is a socket or a vnode respectively,
return the proper errno on error and increase the use count with
.Fn FREF .
.Sh CONCURRENT ACCESS
Since multiple processes can share the same file descriptor table,
it's important that the file is not freed in one process while some
other process is still accessing it.
To solve that problem a special use count is kept with the functions
.Fn FREF
and
.Fn FRELE .
In most cases
.Fn FREF
should be used on a file after it has been extracted
from the file descriptor table and
.Fn FRELE
should be called when the file won't be used anymore.
There are cases when this isn't necessary, but since
.Fn FREF
and
.Fn FRELE
are cheap to use, there is no reason to risk introducing bugs by
not using them.
.Sh CODE REFERENCES
The majority of those functions are implemented in
.Pa sys/kern/kern_descrip.c .
The function prototypes and the macros are located in
.Pa sys/sys/file.h
and
.Pa sys/sys/filedesc.h .
.Sh SEE ALSO
.Xr vnode 9
